The invention relates to a method for forming an address, which method is employed in signal predistortion used for compensating for signal distortions by means of corrector coefficients placed in a corrector table, the corrector coefficients being retrieved from the corrector table on the basis of the address.
The invention also relates to an arrangement for forming a corrector table address used for signal predistortion which is used for compensating for signal distortions by means of corrector coefficients placed in a corrector table, the corrector coefficients being retrieved from the corrector table on the basis of the address.
Many electronic components bring about signal distortion when the signal is processed by the component. In practice, signal distortion is problematic especially in amplifiers. Distortion in the amplifier is caused by the non-linearity of the amplifier. The signal to be amplified can also become distorted because of too strong an input signal or because of for example a shift in the standby operating point of the amplifier. It is possible to reduce distortion for example by a feedback.
Linear modulation methods that enable a spectrum to be used efficiently are becoming more common in mobile systems. However, the linear modulation methods require that the amplifier must be as linear as possible. However, the power amplifiers in use cause intermodulation distortion to the signal. Intermodulation distortion occurs when the amplifier generates harmonic waves, i.e. new frequencies. More precisely, intermodular distortion occurs when an excitation signal comprises oscillation at different frequencies. In that case, the response comprises a number of signal whose frequencies are sums and differences of the excitation frequencies and their multifolds.
Distortion in the amplifier can further depend for example on the magnitude of capacitance and inductance in the amplifier. Mobile systems contain for example AM-AM or AM-PM intermodular distortion (AM=Amplitude Modulation, PM=Phase Modulation). Distortion caused by intermodulation distortion is presented for example by means of a signal state diagram. The signal state diagram can be presented by means of coordinates. The AM-AM distortion causes signal state points to shift closer to the origin. The AM-PM distortion, on the other hand, causes the signal state points to move round the origin.
Various linearization methods are used to correct and compensate for the amplifier-induced signal distortion. Prior art methods employ digital predistortion in linearization. In digital predistortion, a baseband signal is corrected by means of correction coefficients stored in a table. The correction coefficients are retrieved on the basis of an address that has been calculated from an input signal. An input signal amplitude, i.e. the square root of the input signal, is used as the address. However, calculating the square root reduces calculation capacity. In addition, implementing the calculation is complicated.
For example U.S. Pat. No. 5,138,144 presents an error compensation method associated with image reading equipment, the method comprising address information calculations. However, calculating address information is time-based. More precisely, calculating an address is based on a clock pulse.
The object of the present invention is to change the address distribution of a corrector table on the basis of the amplitude distribution of an input signal.
This object is achieved by the method of the type presented in the introduction, characterized by calculating a base address on the basis of a signal, summing up the signal components of a signal composed of signal components, correcting the calculated base address by an address correction value, calculating a corrector table address on the basis of the base address and the corrected base address.
This object is also achieved by the arrangement of the invention, characterized in that the arrangement comprises calculation means that, on the basis of a received signal, calculate a result corresponding to the squaring of the received signal, summing means that, if required, sum up the results corresponding to the squaring, the sum forming a base address, and error correction means that correct the above calculated base address by means of an address correction value.
The method of the invention provides many advantages. The method enables a correction table address to be formed without complicated calculations. According to the method, a base address is formed before forming the actual corrector table address. The actual address is obtained by suitably processing the base address, the actual address then being used as the corrector table address. Processing the base address is carried out by using address correction means that correct the above calculated base address. Processing is carried out by means of address correcting values. The address correcting values arranged to the input signal enable an optimal corrector table address to be formed. The optimal address enables a distortion to be accurately compensated for.